1. Field of the Invention
The present invention relates to micro-switching devices manufactured by MEMS technology, and to a method of manufacturing switching devices by MEMS technology.
2. Description of the Related Art
In the field of radio communications equipment such as mobile telephones, there is an increasing demand for smaller RF circuitry due to the increase of parts needed to be incorporated for providing high performance. In response to such a demand, size reduction efforts are being made for a variety of parts necessary for constituting the circuitry, by using MEMS (micro-electromechanical systems) technology.
MEMS switches are examples of such parts. MEMS switches are switching devices in which each portion is formed by MEMS technology to have minute details, including e.g. at least one pair of contacts which opens and closes mechanically thereby providing a switching action, and a drive mechanism which works as an actuator for the mechanical open-close operations of the contact pair. In switching operations particularly for high-frequency signals in the Giga Hertz range, MEMS switches provide higher isolation when the switch is open and lower insertion loss when the switch is closed, than other switching devices provided by e.g. PIN diode and MESFET because of the mechanical separation achieved by the contact pair and smaller parasitic capacity as a benefit of mechanical switch. MEMS switches are disclosed in e.g. JP-A-2004-1186, JP-A-2004-311394, JP-A-2005-293918, and JP-A-2005-528751.
FIG. 19 through FIG. 23 show a conventional micro-switching device X3. FIG. 19 is a plan view of the micro-switching device X3, and FIG. 20 is a partial plan view of the micro-switching device X3. FIG. 21 through FIG. 23 are sectional views taken in lines XXI-XXI, XXII-XXII and XXIII-XXIII respectively in FIG. 19.
The micro-switching device X3 includes a base substrate S3, a fixing member 31, a movable part 32, a contact electrode 33, a pair of contact electrodes 34 (illustrated in phantom lines in FIG. 20), a driver electrode 35, and a driver electrode 36 (illustrated in phantom lines in FIG. 20).
As shown in FIG. 21 through FIG. 23, the fixing member 31 is bonded to the base substrate S3 via the boundary layer 37. The fixing member 31 and the base substrate S3 are formed of monocrystalline silicon whereas the boundary layer 37 is formed of silicon dioxide.
As shown in FIG. 19, FIG. 20 or FIG. 23 for example, the movable part 32 has a stationary end 32a fixed to the fixing member 31, as well as a free end 32b. The movable part extends along the base substrate S3, and is surrounded by the fixing member 31 via a slit 48. The movable part 32 is formed of monocrystalline silicon.
As shown in FIG. 20 and FIG. 23, the contact electrode 33 is near the free end 32b of the movable part 32. As shown in FIG. 21 and FIG. 23, each contact electrode 34 is formed on the fixing member 31 and has a region facing the contact electrode 33. Also, each contact electrode 34 is connected with a predetermined circuit selected as an object of switching operation, via predetermined wiring (not illustrated). The contact electrodes 33, 34 are formed of a predetermined electrically conductive material.
As shown in FIG. 20 and FIG. 22 for example, the driver electrode 35 is on the movable part 32. Also, the driver electrode 35 is connected with wiring 39 which is laid on the movable part 32 and on the fixing member 31. The driver electrode 35 and the wiring 39 are formed of a predetermined electrically conductive material. The driver electrode 35 and the wiring 39 such as the above are formed by means of thin-film formation technology, and during their formation process, an internal stress develops in the driver electrode 35 and the wiring 39. Because of the internal stress, the driver electrode 35 and the wiring 39, as well as the movable part 32 bonded thereto are warped as shown in FIG. 23. Specifically, the warping or deformation of the movable part 32 causes the free end 32b of the movable part 32 to come closer to the contact electrode 34. The amount of displacement of the free end 32b toward the contact electrode 34 depends on the length and the spring constant of the movable part 32, ranging from 1 through 10 μm approximately.
As shown in FIG. 22, the driver electrode 36 has its ends bonded to the fixing member 31 so as to bridge over the driver electrode 35. Also, the driver electrode 36 is grounded via predetermined wiring (not illustrated). The driver electrode 36 is formed of a predetermined electrically conductive material.
In the micro-switching device X3 arranged as described above, electrostatic attraction is generated between the driver electrodes 35, 36 when an electric potential is applied to the driver electrode 35 via the wiring 39. With the applied electric potential being sufficiently high, the movable part 32, which extends along the base substrate S3, is elastically deformed until the contact electrode 33 makes contact with both of the contact electrodes 34, and thus a closed state of the micro-switching device X3 is achieved. In the closed state, the pair of contact electrodes 34 are electrically connected with each other by the contact electrode 33, to allow an electric current to pass through the contact electrodes 34. In this way, it is possible to achieve an ON state of e.g. a high-frequency signal.
On the other hand, with the micro-switching device X3 assuming the closed state, if the application of the electric potential is removed from the driver electrode 35 whereby the electrostatic attraction acting between the driver electrodes 35, 36 is cancelled, the movable part 32 returns to its natural state, causing the contact electrode 33 to come off the contact electrodes 34. In this way, an open state of the micro-switching device X3 as shown in FIG. 21 and FIG. 23 is achieved. In the open state, the pair of contact electrodes 34 are electrically separated from each other, preventing an electric current from passing through the contact electrodes 34. In this way, it is possible to achieve an OFF state of e.g. a high-frequency signal.
Generally, the driving voltage of a micro-switching device should be low. For micro-switching devices of an electrostatically driven type, the driving voltage can be reduced effectively by reducing the gap between the cooperating driver electrodes. The electrostatic attraction between the driver electrodes is proportional to the square of the distance (gap) between the driver electrodes, which means that the smaller the distance between the driver electrodes, the smaller is the voltage necessary to generate the electrostatic attraction, i.e. the driving force. However, in the conventional micro-switching device X3, it is difficult or even impossible to achieve sufficient reduction in the driving voltage by making small the gap G between the driver electrodes 35, 36.
In the micro-switching device X3, the free end 32b of the movable part 32 comes closer to the contact electrode 34 due to the deformation or warp of the movable part 32, as described above. For this reason, as shown in FIG. 23, the gap G between the driver electrodes 35, 36 when the device is in the non-operating state or the open state becomes wider as the distance from the contact electrodes 33, 34 increases. Specifically, with a distance D1 being the distance between the driver electrodes 35, 36 at a location on the driver electrode 35 on a side farther from the contact electrodes 33, 34, and a distance D2 being the distance between the driver electrodes 35, 36 at a location on the driver electrode 35 on a side closer to the contact electrodes 33, 34, the distance D1 is greater than the distance D2. Referring to FIG. 20, in a case where the driver electrode 35 has a length L1 of 200 μm, the difference between the distance D1 and the distance D2 can sometimes as large as 2 μm. In other words, if the length L4 of the driver electrode 35 is 200 μm, the distance D1 can be larger than the distance D2 by as much as 2 μm even if the distance D2 is made as small as possible. In the driver electrode 35, 36 such as the above, an amount of electrostatic attraction generated at a location of the driver electrode 35 on a side farther from the contact electrodes 33, 34 is substantially smaller than an amount of electrostatic attraction generated at a location of the driver electrode 35 on a side closer to the contact electrodes 33, 34.
As described above, in the micro-switching device X3, the distance D1 is undesirably larger than the distance D2, and therefore it is impossible to make the gap G between the driver electrodes 35, 36 sufficiently small, and as a result, it is sometimes impossible to achieve sufficient reduction in the driving voltage.